Friday, September 27, 2013

This sort of fits under the category of "gee, who knew" but it is nice to finally have confirmation of something that has been tacitly understood for almost ninety years. Science Daily writes:

The study, published in a forthcoming issue of the Journal of Human Evolution, confirms a controversial finding made by anatomist Raymond Dart, who discovered the first known two-legged walking (bipedal) human ancestor, Australopithecus africanus. Since Dart's discovery in 1925, physical anthropologists have continued to debate whether this feature of the cranial base can serve as a direct link to bipedal fossil species.

Okay, first, the controversial nature of this is being massively overstated. There is very little doubt, based on comparative anatomical studies, that the placement of the foramen magnum is directly indicative of the kind of locomotor pattern that a given animal employs. That is what clued Dart in to the whole idea that the Taung child represented a biped in the first place. Its foramen magnum was not in the place that you would have expected it to be if the individual had been either a quadrupedal baboon or extinct ape (of which there were none in South Africa). It was, however, located in the same place as in bipedal humans. Further studies demonstrated that if you placed the Taung skull on a quadrupedal animal, it didn't work. The animal would have its face pointed down all of the time, instead of out, like your average quadruped. Put simply, the Taung child was an early biped and there was no reason to think anything differently.

Once again, comparative anatomical studies to the rescue!

As part of the study, the researchers measured the position of the foramen magnum in 71 species from three mammalian groups: marsupials, rodents and primates. By comparing foramen magnum position broadly across mammals, the researchers were able to rule out other potential explanations for a forward-shifted foramen magnum, such as differences in brain size.

According to the findings, a foramen magnum positioned toward the base of the skull is found not only in humans, but in other habitually bipedal mammals as well. Kangaroos, kangaroo rats and jerboas all have a more forward-shifted foramen magnum compared with their quadrupedal (four-legged walking) close relatives.